Im not really sure about which uses the least cpu power. But most AV programs dont use very much cpu power when they are just running in the backround, unless your actually doing a scan. When scanning on an older computer like yours, things are going to slow down considerably regardless what AV program your using, so your gonna wanna schedule scans for a time when ur not going to be using the computer, while ur sleeping for example. As for the real-time protection effecting performance of your PC you really wanna take a look at how many processes and how much memory the program is using especially if you dont have alot of memory to spare. I have an older notebook I still use which is a Duron 1.2ghz/768mb so I know what its like to have limited resources. I have used a few differant virus programs on this pc, I agree with the above reccomendation for AVG free if you dont wanna spend any money, but there are much better prgrams out there. I have settled on NOD32 AV it has worked well for me and takes up very little resources. Its among the best virus protection you can get and uses very little system resources, only 20mb.